<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
       #out{
          width: 400px;
          height: 400px;
          background-color: blue;  
       }

       #middle{
           width: 200px;
           height: 200px;
           background-color: chartreuse;
       }

       #inner{
           width: 100px;
           height: 100px;
           background-color: violet;
       }


    </style>
</head>
<body>

      <div id='out'>
          <div id='middle'>
              <div id='inner'></div>
          </div>
      </div>
    
</body>
</html>